SEOUL--South Korea on Tuesday blamed North Korea for a cyberattack on nuclear power-plant operator Korea Hydro & Nuclear Power Co., marking the first online incursion publicly attributed to Pyongyang since the hacking of Sony Pictures Entertainment.

South Korean investigators said state-owned Korea Hydro, which operates the country's 23 nuclear reactors, and its business partners were targeted in multiple cyberattacks aimed at stealing internal data that included plant blueprints and employees" personal information.

South Korea's nuclear-plant management wasn't compromised in the attacks and no critical data was disclosed, the investigators said. A series of "spear-phishing" emails aimed at stealing passwords and obtaining remote control access of computers were largely unsuccessful, they added.

A Korea Hydro spokeswoman declined to comment, saying the firm wasn't participating in the investigation.

A Twitter account holder in December posted Internet links to Korea Hydro's internal-data archives and issued various demands to prevent further leaks, the investigators said.

Investigators said they traced the intrusions back to Internet addresses registered by North Korea. The spear-phishing virus that investigators said was used in the attack, named "kimsuky," was previously identified by cybersecurity experts as created in North Korea. The related tweets were posted through servers in Shenyang, in China's northeast, and Vladivostok, Russia, they said.

Pyongyang's state newspaper in late December denied involvement in the cyberattacks, calling such accusations a ploy to escalate inter-Korean tension.

Tuesday's statement was the first time South Korea had publicly attributed the cyberattacks to North Korea.

In the highest-profile cyberattack to date blamed on Pyongyang, private data from Sony Pictures in late November were released online, exposing employees' personal information and email exchanges. A threat posted online and linked to the hackers demanded the studio's cancellation of a film that lampooned North Korea's leader, Kim Jong Un.

The U.S. Federal Bureau of Investigation has said malware found on Sony computers share codes from others previously used by North Korean suspects. North Korea has denied responsibility.

Write to Jeyup S. Kwaak at jeyup.kwaak@wsj.com

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